# How will be the frictional force on a bullet while spinning and non-spinning.

1. Nov 24, 2012

### skoo

when a bullet is travelling at great speeds, how will be the frictional and drag forces on the bullet while considering it spinning and then non-spinning.
to my knowledge i feel that spinning bullet shows less drag and friction than non-spinning, as we know that the rolling friction is always less than static friction.

2. Nov 24, 2012

### K^2

Spinning bullet experiences less drag primarily because spin prevents it from tumbling. If the non-spinning bullet could be forced to remain properly oriented during flight, it would actually experience slightly less drag.

3. Nov 24, 2012

### skoo

if we consider water as the fluid instead of air, would the spinning helps in penetrating through the fluid and shows less drag than that of "non-spinning" ?

4. Nov 24, 2012

### K^2

No. Drag is roughly quadratic with velocity, so you can write its magnitude as kv². Of course, direction will oppose motion. So suppose you want to move along x, and we look at the point on the surface which, due to rotation, also moves along y. Magnitude of this drag is now k(vx²+vy²). The component of this drag along x is given by the following.

$$F_x = -k(v_x^2+v_y^2)\frac{v_x}{\sqrt{v_x^2+v_y^2}} = -k v_x \sqrt{v_x^2 + v_y^2}$$

This value is minimized by vy=0. So spinning increases drag. But if that's the only way to achieve stability, it is usually worth it.

5. Nov 24, 2012

### skoo

but am i wrong if i take the following line as reference..
a screw needs less force than hammering a Nail into a solid block where the resistance is higher than that offered by water!
if not a screw, even in case of nail it will easily penetrate into a semi-solid while twisting due to the Combination of translational kinetic energy and rotational kinetic energy...

6. Nov 24, 2012

### K^2

Completely different issue. Screw is a simple machine. Ignoring losses, it basically allows you to apply smaller force over greater distance doing the same amount of work against wood.

There is a better example. Picture something like a hokey puck that I try to slide across a smooth surface. Given same initial velocity, it will travel further if I give it some spin. Note that I'm not comparing rolling to sliding. The puck slides across the surface in both cases experiencing kinetic friction. But on the puck without the spin, the friction at each contact point is acting against direction of motion. For a spinning puck, the friction points opposite to direction of relative motion at that point, which at some points will be in direction of puck's travel. So net friction will be much smaller, allowing puck to travel further.

This would work for a bullet as well if force due to drag was a constant. But it is not. It scales as square of the velocity, and that's what makes spin generate additional drag instead of reducing it.

7. Nov 25, 2012

### cragar

@ k^2, I still don't completely understand why the spinning puck will go further .
I can see how the rotation at certain ends of the puck will be perpendicular to motion.
The entire puck is still touching the ice. Does it have something to do with the fact that
the spinning has broken it free from static friction.

8. Nov 25, 2012

### K^2

It's free from static friction either way, because it's moving. Think about it this way. Imagine a puck that's just spinning. What is the net friction force. (Not torque, I know there will be torque. Just the force.) It's zero, right. Now imagine a puck that's moving, but it spins so fast that through most of the surface, the overall movement of the puck is practically unimportant. The friction at each point will be almost the same as with the puck spinning without moving otherwise. And that means, friction is almost zero.

Maybe it would help you if you drew yourself a picture. Select some contact points around the puck. Draw friction forces for each one for moving without spinning, just spinning, and moving and spinning. See how these add up to the total.

9. Nov 27, 2012

### JustinRyan

The primary reason for rifling the barrel and imparting spin to the projectile is to even out the drag in the perpendicular plane and keep the bullet on target rather than reducing the drag.